Why not Dia duit, cén aois tú? instead of ... thú?, because I previously met both translations correct? I thought this thú is some emphasis of tú. On the other hand Ó Donaill's dictionary says: tú » thú used as object of vb.). What do they mean?

I could be wrong, but I think it is thú in this sentence because cén, as a contraction of cé and an, hides a copula in it. When a pronoun is the subject of a copula, it uses the disjunctive form (like thú, in this case). Of course, there are other criteria, like when the pronoun is not the subject of a sentence.

All in all, I'm not an expert with Irish grammar (yet!), but I think I have a basic idea of it. Maybe someone can confirm/deny what I said, or elaborate if I wasn't clear.